(a) Field of the Invention
The present invention relates to an automatic transmission, and, more particularly, to a forced downshift control method for an automatic transmission which prevents the generation of shift shock when changing from a power OFF to a power ON state during forced 4-2 downshifting.
(b) Description of the Related Art
Conventional automatic transmissions used in vehicles include a torque converter, a multi-stage gear shift mechanism connected to the torque converter, and a plurality of friction elements actuated by hydraulic pressure for selecting one of the gear stages of the gear shift mechanism, the gear shift mechanism being realized through a planetary gearset. The friction elements are controlled to engaged and disengaged states by a hydraulic control system, which controls hydraulic flow generated by an oil pump, to change shift ratios of the planetary gearset.
The friction elements are selectively operated by a plurality of valves, which undergo port conversion to change the flow of hydraulic pressure, and actuators supplying hydraulic pressure to the valves. Further, a manual valve, indexed with a driver-operated shift select lever to realize port conversion, is connected to a plurality of lines to supply hydraulic pressure from the oil pump to each valve and actuator.
In the automatic transmission structured as in the above, when changing shift modes (P, R, N, D, 2 and L), and when changing into different speeds within the drive D range, the friction elements are changed from a disengaged state to an engaged state and vice versa. A timing of the engagement of specific friction elements in relation to the disengagement of other specific friction elements determines overall shift quality of the automatic transmission. Accordingly, much research has been conducted on shift control methods that better effect such operations.
The different possible shift operations in the drive D range of the automatic transmission in the case where there are a total of four speeds include (a) downshifting successively from a fourth speed to a first speed; (b) upshifting successively from the first speed to the fourth speed; (c) forced downshifts, or kickdown, from the fourth speed to a second speed, and from a third speed to the first speed; and (d) driver-induced skip upshifting from the first to third speed, and the second to fourth speed. The present invention relates to the method of controlling forced downshifts from the fourth speed to the second speed of the drive D range.
When performing a forced 4-2 downshift, the driver abruptly depresses the accelerator pedal in a state of driving in the fourth speed. This causes a sudden rise in throttle pressure, and if road speed is not too high, a forced downshift into the second speed will occur. If the conditions are right for the forced downshift to occur, an end clutch is disengaged, a kickdown servo is maintained engaged, and a rear clutch is controlled to an engaged state, all at the time of changing from a power OFF to a power ON state. In the conventional automatic transmission, if the power state is changed into the power ON state, an end clutch pressure is maintained by open-loop control until a second speed synchronization point is reached.
However, in the power OFF forced 4-2 downshift described above, by maintaining end clutch pressure using open-loop control when changing into the power ON state, operation of the end clutch is continued in a state where operational pressure is being supplied to the rear clutch. This causes shift shock at the end of the shifting operation such that shift quality and ride comfort are reduced.